1. Field of the Invention
The present invention relates to the formation of color images and sets of color images which may be high contrast images including black images on substrates by dry peal-apart imaging processes. Of particular importance to the practice of the present invention is the fabrication of a three or four color image (cyan, yellow, magenta and optional black) on a transparent substrate, particularly in a process for generating a pre-press color proofing image for the printing industry.
2. Background of the Art
In color reproduction, the color accuracy of graphic art color separation negatives are generally verified using color proofing systems prior to making the corresponding printing plates. The color proofing systems must provide a consistent representation of the final color print. It is well known in the art that the halftone dot retention plays a significant role in the color rendition of the final image. Slight changes in the dot reproduction curve can cause significant changes in visual color perception.
Several different types of color proofing systems are well known in the literature. Both digital and conventional proofing systems are available commercially. In digital systems, digitized data is used to directly image the color proofing materials. However, conventional proofing systems are primarily used when verifying the color accuracy of graphic art color separation films. Conventional color proofing systems consist of two types: overlay and surprint.
Overlay systems provide a convenient and quick way to visualize a representative color on a transparent substrate. A typical overlay film construction includes a polyester substrate and photosensitive color layer. Optional protective layers may also be included over the surface of the photosensitive color layer. A color proof is constructed by individually imaging and developing each representative color proofing film. The color imaged films are then laid upon each other in register on a paper stock. Even though this method provides a very simple way of combining two or more colors in registration, it has several inherent disadvantages. The multiplicity of plastic sheets cause incident light to internally reflect which imparts a gloss, color hue shift and a distortion of the final image. Antireflection coatings improve the visual perception of the proof; however, the presence of the thick polyester between each colored image still causes visual distortion of the final image. Examples of these types of constructions may be generally found in U.S. Pat. Nos. 3,136,637; 4,304,836; 4,482,625; 4,634,652; 4,748,101; 4,952,482; and 5,258,261.
A surprint proof is generated by successively superimposing different colored layers upon each other on a single sheet. The surprint color proofing materials are composed of two primary types: photosensitive precolored sheets and photosensitive colorless sheets whose latent image may be toned with pigments, inks or dyes. Some examples of post-colored proofing materials may be found in U.S. Pat. Nos. 3,060,024; 3,634,087; and DE 3,336,431. In the photopolymer systems the image is generally weak and often requires a protective layer. The resultant color hue is also very user dependent due to the dependency upon color density control, lot to lot variation the colorants and dusting of the toners causing color contamination. The electrostatic systems require more sophisticated equipment and environmental control which can be very expensive.
Another example of a precolored proofing system includes photosensitive constructions that are imaged prior to lamination to the receptor or intermediate sheet. Representative examples of these types of systems are described in U.S. Pat. Nos. 4,482,625 and 4,304,836. These systems require the imaged films to be laminated in register which is dependent upon laminating speed and temperature, and dimensional stability of the film and receptor. The accuracy of the registration is limited to smaller format proofs due to the difficulty in retaining perfect alignment of images during the thermal lamination process. Even though the exposure times may be shorter in these systems, productivity is lost due to the difficulties in producing several small proofs on a single large sheet known as scatter proofing.
U.S. Pat. No. 3,671,236 describes a negative acting presensitized color proofing element comprising a carrier sheet having a smooth release surface, a diazo sensitized color layer and a non-photoactive barrier layer upon which may be coated a pressure sensitive adhesive or a thermal adhesive as described in U.S. Pat. No. 4,656,114. The developing media for this system is a 50:50 mixture of n-propanol and water. The barrier layer is present as a carrier for the image and to reduce interaction between the adhesive and the photoactive layer. U.S. Pat. Nos. 4,650,738; 4,751,166; 4,808,508; 4,929,532; and 5,008,174; EP 365,356 and EP 365,357 patent applications describe improvements on this construction by eliminating the need for a barrier layer between the diazo based photoactive layer and the adhesive. The developer used in these systems are aqueous alkali solutions containing greater than 3% surfactant.
U.S. Pat. No. 4,596,757 discloses a further improvement on the construction described in U.S. Pat. No. 3,671,236 by utilizing a photo monomer system in place of the diazonium binders. Again an aqueous alkali developer is used which contains high concentrations of a surfactant (the example cited used 8.5%).
U.S. Pat. No. 5,248,583 describes a construction which utilizes a photoactive color layer containing a photo-oligomer as the photopolymerizable component and a photopolymerizable barrier layer to provide a proofing element that can be developed in a low solids aqueous developer having a pH of 10.2. This construction allows one to develop the image using a more environmentally compatible effluent since the aqueous developer contains less caustic and less ingredients than those used in the previously described systems.
To achieve a system that is developable in a developing solution having a more neutral pH, the binders used in the photosensitive proofing formulation typically make the image more susceptible to attack by the developer. When the resistively developable image is swelled or attacked by the developer it becomes increasingly more difficult to hold the highlight dots (0.5-10% halftone dots) without sacrificing the shadow areas (90-99.5% halftone dots). The shadow areas are sacrificed when the photosensitive layer is overexposed to achieve sufficient cross-link density in the highlight dots to resist developer attack. However, when a photosensitive layer is overexposed, the shadow areas begin to fill-in due to halation effects. This over-exposure also increases the overall dot gain, which significantly affects the color rendition. Therefore, there exists a need for a proofing system that maintains highlight halftone dots without over-exposure and is provided by thermal laser induced imaging processes which do not require adhesive layers on the donor elements.
One method of making colored images upon a non-photosensitive base uses laser induced colorant transfer or laser induced mask production. A donor element is induced (as by ablative levels of radiation) to transfer donor color material to a receptor surface. The donor may contain radiation or thermally sensitive materials to enhance the efficiency of transfer, or the donor material may be ablated without additional materials. Examples of this type of imaging process includes U.S. Pat. Nos. 5,156,938, 5,171,650, 5,256,506, and 5,089,372. The first three patents generally refer to producing a pattern of intelligence.
U.S. Pat. No. 5,171,650 discloses methods and materials for thermal imaging using an "ablation-transfer" technique. The donor element for that imaging process comprises a support, an intermediate dynamic release layer, and an ablative carrier topcoat. The topcoat carries the colorant. The dynamic release layer may also contain infrared-absorbing (light to heat conversion) dyes or pigments. The pigments also include black copper as an additive. Nitrocellulose is disclosed as a possible binder.
Copending U.S. application Ser. No. 07/855,799 discloses ablative imaging elements comprising a substrate coated on a portion thereof with an energy sensitive layer comprising a glycidyl azide polymer in combination with a radiation absorber. Demonstrated imaging sources included infrared, visible, and ultraviolet lasers. Solid state lasers were disclosed as exposure sources, although laser diodes were not specifically mentioned. This application is primarily concerned with the formation of relief printing plates and lithographic plates by ablation of the energy sensitive layer. No specific mention of utility for thermal mass transfer was made.
U.S. Pat. No. 5,308,737 discloses the use of black metal layers on polymeric substrates with gas-producing polymer layers which generate relatively high volumes of gas when irradiated. The black metal (e.g., aluminum) absorbs the radiation efficiently and converts it to heat for the gas-generating materials. It is observed in the examples that in some cases the black metal was eliminated from the substrate, leaving a positive image on the substrate.
U.S. Pat. No. 5,278,023 discloses laser-addressable thermal transfer materials for producing color proofs, printing plates, films, printed circuit boards, and other media. The materials contain a substrate coated thereon with a propellant layer wherein the propellant layer contains a material capable of producing nitrogen (N.sub.2) gas at a temperature of preferably less than about 300.degree. C.; a radiation absorber; and a thermal mass transfer material. The thermal mass transfer material may be incorporated into the propellant layer or in an additional layer coated onto the propellant layer. The radiation absorber may be employed in one of the above-disclosed layers or in a separate layer in order to achieve localized heating with an electromagnetic energy source, such as a laser. Upon laser induced heating, the transfer material is propelled to the receptor by the rapid expansion of gas. The thermal mass transfer material may contain, for example, pigments, toner particles, resins, metal particles, monomers, polymers, dyes, or combinations thereof. Also disclosed is a process for forming an image as well as an imaged article made thereby.
A series of patents (U.S. Pat. Nos. 4,965,242, 4,962,081, 4,975,410, 4,923,860, 5,073,534, and 5,166,126) have been assigned to Kodak disclosing the use of thermal dye diffusion transfer to make filter elements and color filter constructions. U.S. Pat. Nos. 4,965,242 and 5,073,534 teach the use of high T.sub.g polycarbonate and polyester receiving layers to accept the thermally transferred dye. With both receiving layers, a vaporous solvent treatment is required to drive the dye into the receiving layer.
WO 96/10215 (PCT/EP95/03423) describes a method for the formation of a negative color image comprising the following steps in order:
(A) providing a photosensitive element comprising a transparent support, a subbing layer, a photosensitive layer comprising (polymeric binder, colorant, photoinitiator system, and photopolymerizable ethylenically unsaturated monomer, with the weight ration of monomer/binder being at least 4:1), and an adhesive layer comprising a thermoplastic polymer with a Tg between 20.degree. C. and 100.degree. C., PA1 (B) either (i) providing a receiver base and laminating the receiver base at elevated temperature to the adhesive layer and then imagewise exposing the photosensitive layer, or (ii) imagewise exposing said photosensitive element to actinic radiation, and then providing a receiver base and laminating the receiver base at elevated temperature to the adhesive layer, PA1 (C) peeling apart said transparent layer and said receiver base whereby the adhesive layer and the imagewise exposed areas of said photosensitive layer are transferred to the receiver base. Steps (A) through (C) may be repeated to provide multicolor images. PA1 a) a carrier layer, b) a pigmented color photohardenable layer (referred to herein as the "color" layer), c) a photopolymeric adhesive layer (referred to herein as an "Adhesive" layer), and an optionally preferred strippable liner layer. The viscosity of the unexposed photopolymer adhesive layer must be below 100,000 cps at 25.degree. C. to avoid premature delamination of the color layer from the carrier layer.
WO 96/10216 shows processes similar to those of WO 96/10215 except that by control of the monomer to binder ratio, either positive or negative images may be transferred to a receiver base.
UK Patent Application GB 2 129 952 A describes a peel imaging system comprising a first carrier sheet, a photopolymerizable layer (containing a non-attenuating, non-photosensitive dye or pigment and contains material which changes color when exposed to actinic radiation), and a second carrier sheet, at least the first carrier sheet being transparent or translucent, the photopolymerizable layer both polymerizing and changing color upon exposure to actinic radiation. After peeling apart of the carrier, the photopolymerizable material shears along the edges of the imaged areas so that exposed areas of the photopolymerizable layer remain on the first carrier sheet.
U.S. Pat. No. 4,288,525 describes a photosensitive material which may be used in peel-apart imaging processes, particularly in the formation of dry transfer materials, resist masks, printing plates and silk screen stencils. The imaging material consists of two carrier sheets which may be pealed apart and which are laminated together via at least two interlayers (a photosensitive layer and an image forming layer). Prior to exposure, if the carrier sheets are peeled apart, the carrier sheet adjacent the photosensitive layer separates wholly from it, leaving both photosensitive layer and image forming layer on the other carrier sheet. After exposure, if the sheets are peeled apart, the whole of the photosensitive layer and at least part of the image forming layer (according to its imagewise exposure) remains attached to the carrier sheet adjacent the photosensitive layer. By incorporating adhesive in or adjacent the shearable image forming layer, the photosensitive material may constitute a material from which dry transfers may be manufactured simply by exposure and peeling apart.
EPO Application No. EP 0 323 191 A2 describes a peel-apart imaging material useful in image transfer processes. The imaging material comprises a substrate and a photosensitive layer, the photosensitive layer (as two or more layers) comprising a dye or pigment and a photopolymerizable compound. The dye or pigment is contained in at least one of these two or more layers, but is absent from the layer in direct contact with the substrate. Upon imagewise exposure of the image forming material, the exposed and unexposed areas differ in their adhesion to a receptor and image transfer occurs through a breakaway separation within the layer in direct contact with the substrate.
WO 93/03928 describes peel-apart elements for laser induced thermal imaging processes, the elements comprising, in order, a support, an active layer (infrared absorbing material, and polymeric binder), an adhesive layer, and a cover sheet. The adhesive interrelationship between layers are such that upon peeling the layers apart, regions of the active layer exposed with infrared radiation adhere to the support while exposed regions adhere to the adhesive layer and the cover sheet, forming a negative image and a positive image on the respective sheets.
EPO Applications 0 096 572 A2; 0 385 466 A2; and 0 601 760 A2 and U.S. Pat. Nos. 4,050,936; 4,347,300; 4,291,114; 5,001,036; 5,234,790; 5,300,399; and 5,374,780 also describe peel-apart photosensitive imaging systems with adhesive layers.